1. Field of the Invention
The present invention relates to a data transfer method when transmitting and receiving data via a wireless communications network and a mobile node and a relay node that are used in the wireless communication network.
2. Description of the Related Art
FIG. 1 is a structure example of an existing network. A home agent (a mobile-supporting node, HA) 2 which is a relay node, access routers 4 and 5, and a router 6 are connected to the Internet 10. Suppose that a corresponding node (CN) 3 under a network C is transmitting data to a mobile node 1 through a relay node 2. Further, the mobile node 1 is under a wireless network A and is communicating with the corresponding node 3 through the access router 4. After the mobile node 1 becomes under a contiguous wireless network B due to the move or the like, it communicates through the access router 5.
FIG. 2 is a diagram explaining the operation at the time of a hand over occurrence.    (1) The mobile node 1 receives a router advertisement RA 1 from the access router 4.    (2) The mobile node 1 creates a care of address CoA-1.    (3) The mobile node 1 makes a location registration to the home agent 2.    (4) The home agent 2 relates a home address of the mobile node 1: HoA-1 and the care of address CoA-1 using a binding cache, and manages information that the mobile node 1 is currently under the wireless network A.    (5) The mobile node 1 moves to the wireless network B under the access router 5 from the wireless network A under the access router 4, and a hand over occurs.    (6) The mobile node 1 receives a router advertisement RA 2 from the access router 5.    (7) The mobile node 1 creates a care of address CoA-2.    (8) The mobile node 1 makes a location registration to the home agent 2.    (9) The home agent 2 relates the home address of the mobile node 1: HoA-1 and the care of address CoA-2 created by the mobile node 1 at step (7), using the binding cache of the mobile node 1, and manages them.
FIG. 3 is a diagram explaining a problem of a conventional technique when a hand over occurs. Since the operations in steps (1) to (4) in FIG. 3 are the same as in steps (1) to (4) in FIG. 2 respectively, a description thereof is omitted here and operations in steps from (5) onward will be described below.    (5) The mobile node 1 requests a streaming distribution from the corresponding node 3.    (6) The corresponding node 3 executes a process of the streaming distribution.    (7) A packet of the streaming distribution is transmitted to the mobile node 1 from the corresponding node 3.    (8) During the streaming distribution, the mobile node 1 moves to the wireless network B from the wireless network A, and a hand over occurs.
Here, the operations in steps (9) to (12) are corresponding to those in steps (6) to (9) in FIG. 2. With the operations in steps (9) to (12) in FIG. 3, the mobile node 1 under the wireless network B makes a location registration, and the home agent 2 relates the home address HoA-1 and the care of address CoA-2 and managed them.    (13) The mobile node 1 receives streaming information transmitted from the corresponding node 3.
In this case, the streaming information distributed in steps (9) to (12) could not be received by the mobile node 1. If there is a packet that is not received in these steps, a re-transmission processing is performed by the TCP protocol and it brings about a problem that network efficiency decreases.
The problem in the conventional technique in FIG. 3 will be described with reference to the sequence when a hand over occurs in FIG. 4. Each number indicated in FIG. 4 corresponds to each operation in FIG. 2. The mobile node 1 under the wireless network A, which is communicating through the access router 4, is receiving a packet of the streaming distribution from the corresponding node 3 at step (7), after requesting the streaming distribution at step (5). For simplification of the diagram, the data transmission of the streaming distribution from the corresponding node 3 to the mobile node 1 is shown only for a single packet. In practice, a plurality of packets are transmitted to the mobile node 1.
Here, the mobile node 1 moves to the wireless network B from the wireless network A in step (8). The mobile node receives a router advertisement from the access router 5 and makes a location registration. After the operations in steps (9) to (12), a packet of which destination is the CoA-2 is transmitted from the corresponding node 3. However, the streaming distribution continues to be made to the CoA-1 that is the destination before moving until the operations in steps (9) to (12) are completed. Therefore, since the packet could not be received by the mobile node 1, there arises a problem that packet loss could occur.
As a method for preventing the packet loss, there is a method of making the home agent of the relay node store the information during the hand over in the home agent using buffering in order to prevent the packet loss from occurring and making the mobile node resume receiving data from the first cell among the data that has not been received after the completion of the hand over (patent document 1, for example). Further, in the case of a hierarchical network, a method for specifying an optimum buffering point is disclosed (patent document 2, for example). A method for preventing the packet loss in the case where the home agent of the relay node is provided with a function of the buffering will be described with reference to FIG. 5.
FIG. 5 is a diagram explaining the operation of flow control when a conventional hand over occurs. Steps (1) to (6) in FIG. 5 are the same as in FIG. 3. Steps from (7) onward are shown as follows.    (7) The corresponding node 3 transmits information to be distributed.    (8) As for the information to be distributed, the home agent 2 assigns a sequence number to an outer header of an encapsulated packet to perform the packet buffering.    (9) The home agent 2 transmits the packet to the mobile node 1.    (10) The mobile node 1 retains and manages the sequence number of the received packet.
A hand over occurs at step (11). The mobile node 1 gives the sequence number of the received packet to data to be transmitted in step (14); however, the other steps (11) to (15) are the same as steps (6) to (9) in FIG. 2.    (16) Based on the data transmitted from the mobile node 1 in step (14), the home agent 2 transmits the packet having the corresponding sequence number out of buffered packets in step (8) to the mobile node 1.
After this, in the same manner as steps (8) and (9) in the wireless network A, in the wireless network B to which the mobile node moves, the home agent 2 transmits a packet to the mobile node 1 after assigning a sequence number to the packet to be transmitted and performing the packet buffering, in steps (17) to (19).
[Patent document 1] Japanese Unexamined Patent Publication No. 9-186704 (abstract, FIG. 1, paragraphs 0016 to 0018)
[Patent document 2] Japanese Unexamined Patent Publication No. 2003-69617 (abstract, FIG. 1, paragraphs 0011 and 0012)
In the method of preventing the packet loss by giving the sequence number to the packet, the home agent 2 needs to equally perform the buffering of all the packets distributed from the corresponding node 3 and prepare for the hand over or the like. The mobile node 1 also has to always manage the sequence number of the received packet. Due to this, in the conventional method, there has been a problem that throughput of the home agent or of the mobile node of a receiving node decreases.
Further, in such a case where there exist a plurality of communication destinations and there is a communication among them using a communication application or the like that is inadequate for the buffering, since the buffering is performed for all the communication data, there has been a problem that the process becomes slower.
Moreover, in such a case where a bandwidth of a network after the hand over of the mobile node 1 is narrow, throughput of transmitting the buffered packet to the mobile node 1 may decrease and the amount of packets to be buffered may exceed the buffer size in the home agent 2. There has been a problem that packets which fail to be buffered due to exceeding the buffer size or missing packets due to the change of radio wave strength in a wireless network cannot be compensated in this method.